Links

Reference Data: Real World Elevations of Tidal Datums in Buzzards Bay

In our areas tides are "semi-diurnal." That is to say, there are two high tides and two low tides each day. More precisely, our tides are "mixed semi-diurnal." That is, each of the high and low tides are not of the same elevation. For this reason, tides at a site can be characterized by not only the average elevations of mean high water (MHW) and mean low water (MLW), but a site can be characterized by the elevations of mean higher high water (MHHW) and mean lower low water (MLLW) as well

There is often a need to record on engineering plans or properties the real world elevation of tides, or the elevation of structures relative to tides. Most typically, plans may show the MHW line (the average or all high tides), or the high tide line (HTL, which is the highest tide of the year, also nicknamed the king tide). On our Tidal Datums and Benchmarks page we explain the technical details and theory as to how this is achieved, but for any given site, there are practical obstacles as to how these elevations are precisely defined. This is due in part to the fact that it is difficult to transfer tidal elevations to real world elevations around Buzzards Bay because there are relatively few tidal stations with elevations tied to a real world benchmarks. The problem is exacerbated by the fact that the tidal range in some areas of Buzzards Bay is two feet higher than other areas. Even local mean sea level varies around Buzzards Bay (although differences around Buzzards Bay are less than 3 inches).

To help reconcile tidal data to real world benchmarks, NOAA developed the VDatum computer model. This program enables estimates of real world elevations of tidal datums with good accuracy. In the maps below we show the elevation of local mean sea level (MSL), the great diurnal tidal range (abbreviated GT), and the elevations of mean higher high water (MHHW) around Buzzards Bay. These maps were produced in ArcGIS with elevation estimates from NOAA's VDatum 3.2 model (March 2013).

Real world elevation (in feet) of Local Mean Sea Level in Buzzards Bay. Click the map to enlarge it.